Products made from paper webs such as bath tissues, facial tissues, paper towels, industrial wipers, food service wipers, napkins, medical pads and other similar products are designed to include several important properties. For example, the product should have a relatively soft feel and, for most applications, should be highly absorbent. The product should also have stretch characteristics and should resist tearing in the environment in which they are used. The above properties are especially important when trying to develop a disposable paper wiping product that can serve as a substitute for conventional cloth products.
In the past, various paper wiping products have been developed. For instance, in order to optimize various properties, in the past two or more embossed conventional paper webs have been laminated together. The present invention is directed to further improvements in such laminated wiping products. In particular, the present invention is directed to a wiping product made from separate plies that has different surface characteristics on each side of the product as will be described in greater detail below.
In general, the present invention is directed to a paper wiping product made from at least two plies. The two outer plies of the product have different properties for producing a laminate that has different surface characteristics on each side of the product. In particular, the wiping product of the present invention includes a rough or textured side and a smooth and soft side. The product can be used in numerous applications, particularly to clean or polish any adjacent surface or object.
In one embodiment, the paper wiping product includes a first outer ply made from an uncreped, throughdried paper web. The first outer ply can have a basis weight of from about 15 gsm to about 80 gsm, and in one embodiment, from about 20 gsm to about 27 gsm. The first outer ply includes a textured surface having an Overall Surface Depth of greater than about 0.1 mm, and particularly greater than about 0.2 mm. The first outer ply can contain softwood fibers, hardwood fibers and high-yield fibers. For example, in one embodiment, the first outer ply includes high-yield fibers in an amount up to about 30% by weight in combination with softwood fibers. The density of the first outer ply can be less than about 0.3 g/cm3.
The paper wiping product includes a second outer ply bound or laminated to the first outer ply. The second outer ply is generally softer and smoother than the first outer ply. The second outer ply has a basis weight less than the first outer ply and can be made from softwood fibers, hardwood fibers and high-yield fibers. In one embodiment, the second outer ply is made from a combination of high-yield fibers and softwood fibers and/or hardwood fibers.
The second outer ply can be made from various paper making processes. For instance, the second outer ply can be creped or uncreped. The second outer ply can also be throughdried or can be dried on a heated cylinder or drum, such as a felted Yankee drum dryer.
The first outer ply can be laminated to the second outer ply by any suitable process. For example, in one embodiment, the first outer ply and the second outer ply are embossed and nested together. Alternatively, one or both of the plies are embossed and secured together in an pin-to-pin or random pin-to-pin (unnested) arrangement. As used herein, a xe2x80x9cpin-to-pinxe2x80x9d arrangement refers to laminating together two embossed plies in which the raised or embossed areas of each ply contact each other.
Various binder materials, such as adhesives, can be used to secure the layers together. The adhesive can be, for instance, a polyvinyl alcohol, an acetate or a starch adhesive. Alternatively, other binder materials, such as binder fibers, can be inserted between the plies and heated causing the plies to attach together. The binder fibers can be made from, for instance, any suitable thermoplastic polymer, such as polyester, polyethylene or polypropylene. In one embodiment, the binder fibers can be bicomponent fibers. For some applications, the first outer ply and the second outer ply should be bonded together over the entire surface area of the adjacent plies. Alternatively, the binder materials can be applied at selected areas for attaching the plies together. For example, the binder material can be applied to the raised areas on one of the plies.
The binder material can be applied to the plies according to any suitable process. For example, the adhesive, such as a latex adhesive, can be sprayed or printed onto the plies. Binder fibers, however, can be sprayed onto the plies or applied by other means. For example, in one embodiment, an air forming system can be used to apply the binder fibers in between the adjacent layers. Other features, and aspects of the present invention are discussed in greater detail below.
As used herein, xe2x80x9cdry bulkxe2x80x9d is measured with a thickness gauge having a circular platen 3 inches in diameter such that a pressure of 0.05 psi is applied to the sample, which should be conditioned at 50% relative humidity and at 73xc2x0 F. for 24 hours prior to measurement. The webs can have a dry bulk of 3 cc/g or greater. The uncreped throughdried webs can have a dry bulk of 6 cc/g or greater, particularly 9 cc/g or greater, and more particularly between 8 cc/g and 28 cc/g.
As used herein, xe2x80x9chigh-yield pulp fibersxe2x80x9d are those papermaking fibers produced by pulping processes providing a yield of about 65 percent or greater, more specifically about 75 percent or greater, and still more specifically from about 75 to about 95 percent. Yield is the resulting amount of processed fiber expressed as a percentage of the initial wood mass. Such pulping processes include bleached chemithermomechanical pulp (BCTMP), chemithermomechanical pulp (CTMP) pressure/pressure thermomechanical pulp (PTMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high-yield sulfite pulps, and high-yield kraft pulps, all of which leave the resulting fibers with high levels of lignin. High-yield fibers are well known for their stiffness (in both dry and wet states) relative to typical chemically pulped fibers. The cell wall of kraft and other non-high-yield fibers tends to be more flexible because lignin, the xe2x80x9cmortarxe2x80x9d or xe2x80x9cgluexe2x80x9d on and in part of the cell wall, has been largely removed. Lignin is also nonswelling in water and hydrophobic, and resists the softening effect of water on the fiber, maintaining the stiffness of the cell wall in wetted high-yield fibers relative to kraft fibers. The preferred high-yield pulp fibers can also be characterized by being comprised of comparatively whole, relatively undamaged fibers, high freeness (250 Canadian Standard Freeness (CSF)or greater, more specifically 350 CFS or greater, and still more specifically 400 CFS or greater), and low fines content (less than 25 percent, more specifically less than 20 percent, still more specifically less that 15 percent, and still more specifically less than 10 percent by the Britt jar test).
xe2x80x9cNoncompressive dryingxe2x80x9d refers to drying methods for drying cellulosic webs that do not involve compressive nips or other steps causing significant densification or compression of a portion of the web during the drying process. Such methods include through air drying; air jet impingement drying; non-contacting drying such as air flotation drying, as taught by E. V. Bowden, E. V., Appita J., 44(1): 41 (1991); through flow or impingement of superheated steam; microwave drying and other radio frequency or dielectric drying methods; water extraction by supercritical fluids; water extraction by nonaqueous, low surface tension fluids; infrared drying; drying by contact with a film of molten metal; and other methods. It is believed that the three-dimensional basesheets of the present invention could be dried with any of the above mentioned noncompressive drying means without causing significant web densification or a significant loss of their three-dimensional structure and their wet resiliency properties. Standard dry creping technology is viewed as a compressive drying method since the web must be mechanically pressed onto part of the drying surface, causing significant densification of the regions pressed onto the heated Yankee cylinder.
xe2x80x9cOverall Surface Depthxe2x80x9d. A three-dimensional basesheet or web is a sheet with significant variation in surface elevation due to the intrinsic structure of the sheet itself. As used herein, this elevation difference is expressed as the xe2x80x9cOverall Surface Depth.xe2x80x9d The basesheets useful for this invention possess three-dimensionality and have an Overall Surface Depth of about 0.1 mm. or greater, more specifically about 0.3 mm. or greater, still more specifically about 0.4 mm. or greater, still more specifically about 0.5 mm. or greater, and still more specifically from about 0.4 to about 0.8 mm.
The three-dimensional structure of a largely planar sheet can be described in terms of its surface topography. Rather than presenting a nearly flat surface, as is typical of conventional paper, the molded sheets useful in producing the present invention have significant topographical structures that, in one embodiment, may derive in part from the use of sculptured throughdrying fabrics such as those taught by Chiu et al. in U.S. Pat. No. 5,429,686, which is incorporated by reference. The resulting basesheet surface topography typically comprises a regular repeating unit cell that is typically a parallelogram with sides between 2 and 20 mm in length. For wetlaid materials, it is preferred that these three-dimensional basesheet structures be created by molding the moist sheet or be created prior to drying, rather than by creping or embossing or other operations after the sheet has been dried. In this manner, the three-dimensional basesheet structure is more likely to be well-retained upon wetting, helping to provide high wet resiliency and to promote good in-plane permeability. For air-laid basesheets, the structure may be imparted by thermal embossing of a fibrous mat with binder fibers that are activated by heat. For example, an air-laid fibrous mat containing thermoplastic or hotmelt binder fibers may be heated and then embossed before the structure cools to permanently give the sheet a three-dimensional structure.
In addition to the regular geometrical structure imparted by the sculptured fabrics and other fabrics used in creating a basesheet, additional fine structure, with an in-plane length scale less than about 1 mm, can be present in the basesheet. Such a fine structure can stem from microfolds created during differential velocity transfer of the web from one fabric or wire to another prior to drying. Some of the materials of the present invention, for example, appear to have fine structure with a fine surface depth of 0.1 mm or greater, and sometimes 0.2 mm or greater, when height profiles are measured using a commercial moirxc3xa9 interferometer system. These fine peaks have a typical half-width less than 1 mm. The fine structure from differential velocity transfer and other treatments may be useful in providing additional softness, flexibility, and bulk. Measurement of the surface structures is described below.
An especially suitable method for measurement of Overall Surface Depth is moirxc3xa9 interferometry, which permits accurate measurement without deformation of the surface. For reference to the materials of the present invention, surface topography should be measured using a computer-controlled white-light field-shifted moirxc3xa9 interferometer with about a 38 mm field of view. The principles of a useful implementation of such a system are described in Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991). A suitable commercial instrument for moirxc3xa9 interferometry is the CADEYES(copyright) interferometer produced by Medar, Inc. (Farmington Hills, Mich.), constructed for a 38-mm field-of-view (a field of view within the range of 37 to 39.5 mm is adequate). The CADEYES(copyright) system uses white light which is projected through a grid to project fine black lines onto the sample surface. The surface is viewed through a similar grid, creating moirxc3xa9 fringes that are viewed by a CCD camera. Suitable lenses and a stepper motor adjust the optical configuration for field shifting (a technique described below). A video processor sends captured fringe images to a PC computer for processing, allowing details of surface height to be back-calculated from the fringe patterns viewed by the video camera.
In the CADEYES moirxc3xa9 interferometry system, each pixel in the CCD video image is said to belong to a moirxc3xa9 fringe that is associated with a particular height range. The method of field-shifting, as described by Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991) and as originally patented by Boehnlein (U.S. Pat. No. 5,069,548, herein incorporated by reference), is used to identify the fringe number for each point in the video image (indicating which fringe a point belongs to). The fringe number is needed to determine the absolute height at the measurement point relative to a reference plane. A field-shifting technique (sometimes termed phase-shifting in the art) is also used for sub-fringe analysis (accurate determination of the height of the measurement point within the height range occupied by its fringe). These field-shifting methods coupled with a camera-based interferometry approach allows accurate and rapid absolute height measurement, permitting measurement to be made in spite of possible height discontinuities in the surface. The technique allows absolute height of each of the roughly 250,000 discrete points (pixels) on the sample surface to be obtained, if suitable optics, video hardware, data acquisition equipment, and software are used that incorporates the principles of moirxc3xa9 interferometry with field-shifting. Each point measured has a resolution of approximately 1.5 microns in its height measurement.
The computerized interferometer system is used to acquire topographical data and then to generate a grayscale image of the topographical data, said image to be hereinafter called xe2x80x9cthe height map.xe2x80x9d The height map is displayed on a computer monitor, typically in 256 shades of gray and is quantitatively based on the topographical data obtained for the sample being measured. The resulting height map for the 38-mm square measurement area should contain approximately 250,000 data points corresponding to approximately 500 pixels in both the horizontal and vertical directions of the displayed height map. The pixel dimensions of the height map are based on a 512xc3x97512 CCD camera which provides images of moirxc3xa9 patterns on the sample which can be analyzed by computer software. Each pixel in the height map represents a height measurement at the corresponding x- and y-location on the sample. In the recommended system, each pixel has a width of approximately 70 microns, i.e. represents a region on the sample surface about 70 microns long in both orthogonal in-plane directions). This level of resolution prevents single fibers projecting above the surface from having a significant effect on the surface height measurement. The z-direction height measurement must have a nominal accuracy of less than 2 microns and a z-direction range of at least 1.5 mm. (For further background on the measurement method, see the CADEYES Product Guide, Medar, Inc., Farmington Hills, Mich., 1994, or other CADEYES manuals and publications of Medar, Inc.)
The CADEYES system can measure up to 8 moirxc3xa9 fringes, with each fringe being divided into 256 depth counts (sub-fringe height increments, the smallest resolvable height difference). There will be 2048 height counts over the measurement range. This determines the total z-direction range, which is approximately 3 mm in the 38-mm field-of-view instrument. If the height variation in the field of view covers more than eight fringes, a wrap-around effect occurs, in which the ninth fringe is labeled as if it were the first fringe and the tenth fringe is labeled as the second, etc. In other words, the measured height will be shifted by 2048 depth counts. Accurate measurement is limited to the main field of 8 fringes.
The moirxc3xa9 interferometer system, once installed and factory calibrated to provide the accuracy and z-direction range stated above, can provide accurate topographical data for materials such as paper towels. (Those skilled in the art may confirm the accuracy of factory calibration by performing measurements on surfaces with known dimensions.) Tests are performed in a room under Tappi conditions (73xc2x0 F., 50% relative humidity). The sample must be placed flat on a surface lying aligned or nearly aligned with the measurement plane of the instrument and should be at such a height that both the lowest and highest regions of interest are within the measurement region of the instrument.
Once properly placed, data acquisition is initiated using Medar""s PC software and a height map of 250,000 data points is acquired and displayed, typically within 30 seconds from the time data acquisition was initiated. (Using the CADEYES(copyright) system, the xe2x80x9ccontrast threshold levelxe2x80x9d for noise rejection is set to 1, providing some noise rejection without excessive rejection of data points.) Data reduction and display are achieved using CADEYES(copyright) software for PCs, which incorporates a customizable interface based on Microsoft Visual Basic Professional for Windows (version 3.0). The Visual Basic interface allows users to add custom analysis tools.
The height map of the topographical data can then be used by those skilled in the art to identify characteristic unit cell structures (in the case of structures created by fabric patterns; these are typically parallelograms arranged like tiles to cover a larger two-dimensional area) and to measure the typical peak to valley depth of such structures. A simple method of doing this is to extract two-dimensional height profiles from lines drawn on the topographical height map which pass through the highest and lowest areas of the unit cells. These height profiles can then be analyzed for the peak to valley distance, if the profiles are taken from a sheet or portion of the sheet that was lying relatively flat when measured. To eliminate the effect of occasional optical noise and possible outliers, the highest 10% and the lowest 10% of the profile should be excluded, and the height range of the remaining points is taken as the surface depth. Technically, the procedure requires calculating the variable which we term xe2x80x9cP10,xe2x80x9d defined at the height difference between the 10% and 90% material lines, with the concept of material lines being well known in the art, as explained by L. Mummery, in Surface Texture Analysis: The Handbook, Hommelwerke GmbH, Mxc3xchlhausen, Germany, 1990. In this approach, which will be illustrated with respect to FIG. 7, the surface 31 is viewed as a transition from air 32 to material 33. For a given profile 30, taken from a flat-lying sheet, the greatest height at which the surface beginsxe2x80x94the height of the highest peakxe2x80x94is the elevation of the xe2x80x9c0% reference linexe2x80x9d 34 or the xe2x80x9c0% material line,xe2x80x9d meaning that 0% of the length of the horizontal line at that height is occupied by material. Along the horizontal line passing through the lowest point of the profile, 100% of the line is occupied by material, making that line the xe2x80x9c100% material linexe2x80x9d 35. In between the 0% and 100% material lines (between the maximum and minimum points of the profile), the fraction of horizontal line length occupied by material will increase monotonically as the line elevation is decreased. The material ratio curve 36 gives the relationship between material fraction along a horizontal line passing through the profile and the height of the line. The material ratio curve is also the cumulative height distribution of a profile. (A more accurate term might be xe2x80x9cmaterial fraction curve.xe2x80x9d)
Once the material ratio curve is established, one can use it to define a characteristic peak height of the profile. The P10 xe2x80x9ctypical peak-to-valley heightxe2x80x9d parameter is defined as the difference 37 between the heights of the 10% material line 38 and the 90% material line 39. This parameter is relatively robust in that outliers or unusual excursions from the typical profile structure have little influence on the P10 height. The units of P10 are mm. The Overall Surface Depth of a material is reported as the P10 surface depth value for profile lines encompassing the height extremes of the typical unit cell of that surface. xe2x80x9cFine surface depthxe2x80x9d is the P10 value for a profile taken along a plateau region of the surface which is relatively uniform in height relative to profiles encompassing a maxima and minima of the unit cells. Measurements are reported for the most textured side of the basesheets of the present invention, which is typically the side that was in contact with the throughdrying fabric when air flow is toward the throughdryer. FIG. 8 represents a profile of Example 13 of the present invention, discussed below, having an Overall Surface Depth of about 0.5.
Overall Surface Depth is intended to examine the topography produced in the tissue web, especially those features created in the sheet prior to and during drying processes, but is intended to exclude xe2x80x9cartificiallyxe2x80x9d created large-scale topography from dry converting operations such as embossing, perforating, pleating, etc. Therefore, the profiles examined should be taken from unembossed regions if the tissue web has been embossed, or should be measured on an unembossed tissue web. Overall Surface Depth measurements should exclude large-scale structures such as pleats or folds which do not reflect the three-dimensional nature of the original basesheet itself. It is recognized that sheet topography may be reduced by calendering and other operations which affect the entire basesheet. Overall Surface Depth measurement can be appropriately performed on a calendered basesheet.